Support post processing with `--watch`

[valencik]

TLDR: I would like to invoke some external command every time mdoc finishes generating files while in file watching mode.

Motivation:

I use mdoc to generate typesafe markdown files, and then use pandoc to turn the markdown into html (or any other format pandoc supports). The current workflow looks like:

coursier launch org.scalameta:mdoc_2.13:2.2.10 -- --in hello.md --out mhello.md \
  && pandoc --from=markdown --to=html5 mhello.md --output=hello.html

I'd prefer to be able to leverage mdoc's file watcher mode (--watch) to gain the performance benefits.

I had assumed this would require the life cycle code in #245 but perhaps not?

[olafurpg]

Thank you for reporting! I can see how this would be useful. This feature is unrelated to #245, that PR is about managing the life-cycle of custom modifiers.

I think the best way to implement this feature would be to add some kind of "mdoc event protocol" akin to Bazel's "build event protocol" (https://docs.bazel.build/versions/master/build-event-protocol.html#consuming-the-build-event-protocol). Users could hook into the stream of mdoc events with a service provider. Rough sketch of the API

final class EventHandlerContext private[mdoc] (
  val workingDirectory: Path
  // other useful information
)
final class OnWatchIterationComplete private[mdoc] (
  val updatedFiles: List[Path]
)
abstract class EventHandler {
  def withContext(params: EventHandlerContext): EventHandler = this
  def onWatchIterationComplete(params: OnWatchIterationComplete): Unit = ()
  // We can add new hooks in the future as long as they have a default implementation
}
object EventHandler {
  def default(): List[EventHandler] = default(this.getClass.getClassLoader)
  def default(classLoader: ClassLoader): List[EventHandler] =
    ServiceLoader.load(classOf[EventHandler], classLoader).iterator().asScala.toList
}

The watch event can be sent to handlers around this method here

mdoc/mdoc/src/main/scala-2/mdoc/internal/cli/MainOps.scala

Line 189 in 536b29d

def handleWatchEvent(event: DirectoryChangeEvent): Unit = {

Does this make sense? WDYT?